1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of high-speed printing, which accommodates a large amount of recording sheets at a front-access type sheet feed portion.
2. Description of the Related Art
Recently, with a view to make an apparatus compact in order to reduce the occupied area thereof, an image forming apparatus provided with a front-access type sheet feed portion is under development (see Japanese Unexamined Patent Application No. 10-129856). A front-access type image forming apparatus includes a document reading section (scanner portion) arranged at an upper portion, an operation section arranged at a front of the document reading section, an image forming section arranged at a below the document reading section, and a sheet feed section having a sheet cassette, which accommodates recording sheets necessary for printing, and arranged at a lowermost part for transporting the sheet one by one to the image forming section, for example. In the image forming apparatus of this type, a sheet is accommodated and replenished with the sheet cassette drawn out of the apparatus, and after the replenishment is finished, the sheet cassette is pushed into the apparatus to be reloaded. A sheet placing plate is provided at a bottom part of the sheet cassette. An upward movement of the sheet placing plate raises an uppermost sheet to a sheet feeding point, and a pickup roller is lowered to the sheet feeding point from an upper stand-by position for transporting the sheet one by one to the image forming section by the pickup roller.
In some of the front-access type image forming apparatuses, a separation pawl (regulating member) for regulating a stacking amount and an uppermost surface of the accommodated sheets is provided at an end edge of the sheet cassette at the downstream side in a sheet transporting direction (see Japanese Unexamined Patent Application No. 2004-307183).
For the high-speed printing of the image forming apparatus, a sheet feed section with a larger capacity is now being developed. In order to provide a large capacity to the sheet feed section, since it is not acceptable to raise the positions of the document reading section and the operation section because this causes an inconvenience for a user, it is necessary to devise a way for accommodating many sheets as possible in the sheet cassette. For example, it is considered that the regulation member provided at the conventional sheet cassette is removed so that the sheets can be accommodated to a full height that is generally equal to the height of the sheet cassette, specifically the sheets can be accommodated up to the top height of a side plate that regulates a sheet setting area provided at a bottom part of the sheet cassette.
However, when the sheets S are fully replenished to the sheet cassette 211 drawn out of a main body 200 of the apparatus and the sheet cassette 211 is pushed strongly to be reloaded into the main body 200 with the regulating member for regulating the uppermost surface of the sheets eliminated as shown in FIG. 7A, the uppermost sheet S might lifts up to run on to the side plate 212 as shown in FIG. 7B. In this case, misalignment in printing or jam during transportation occur due to skew conveyance. Notably, reference numeral 220 denotes a pickup roller in FIG. 7.
As a measure to prevent the misalignment of the sheet described above, it is considered that, during the loading process of the sheet cassette, the pickup roller is moved down to press the uppermost sheet. In this case, it is necessary that a stack of sheets is moved up so that the uppermost surface of the sheet is brought to a position (e.g., sheet feeding point) higher than the sheet cassette and the side plate, so as to prevent the descending pickup roller from contacting the sheet cassette. However, this method entails problems as described below. That is, the sheet pressed by the pickup roller may be damaged or wrinkled, resulting in degradation of image quality caused by printing performed on the damaged sheet, or the movement of the sheets positioned at a position higher than the side plate is stopped before completion of the loading to cause misalignment with respect to the sheet cassette, thereby producing a misalignment in printing.